Dear All, I'm not part of your community, but I've been at the APS synchrotron enough to know that you might have the information I'm looking for, and willing to share it. My question is in the subject line. The reason for this question is an upcoming measurement of the ionization energy shift in a hot plasma. We have chosen Ta, because its Lbeta-2 line happens to be slightly lower in energy than the nominal energy of the Zn K-edge. Hence, a small increase, perhaps as low as 1 eV, should produce a substantial change in the attenuation of a sufficiently thick K-edge filter. Crystal spectrometers of the type we use rarely have a resolution that exceeds 2000, so that for a 10 keV l-line we could as best see a 5 eV energy change. With the same approach we could measure a ~10 eV shift on the ~63,000 eV Kalpha_2 line of Ir, with a K-edge filter of Lu. It so happens that their nominal energies are 27 eV apart (according to the x-ray data booklet, not according to Deslattes et al, 2001), so that the line and the K-ege overlap. Both are life time broadened by about 40 eV, so as the energy of the line increases, the transmission of a filter decreases. We first separate the two Kalpha lines (which are 1.3 keV apart), and then measured the decrease in the intensity behind the filter. The idea is to preferentially absorb the high-energy side of the Ir Kalpha_2 line, so that an increase in the line center becomes a decrease in line intensity behind the filter. I appreciate any comment you may have on this matter. Thank you, Nino